Two-part components are widely used in many industries for many purposes, including:
1) Model making: Within the automotive, aerospace, rail, wind turbines energy fields and marine industries there is a need to produce dimensionally accurate master models, particularly of large format. These models are used by engineers for the conceptual design of the individual components utilised in the final product. More and more, such models are tested for technical and functional use, thus requiring technical material properties.
U.S. Pat. No. 5,707,477 and U.S. Pat. No. 5,773,047 describe a method for making prepreg parts for use in the aerospace industry where pliable solid patties prepared from syntactic epoxy material are hand-applied to a block made by stacking successive layers of aluminium honeycomb core. The entire resulting structure is then heated to effect cure of the patties. However, this approach is again labour intensive, in that it involves hand application of the pliable solid patties to the honeycomb core. It also requires heating of the entire structure in order to cure the applied patties. The resulting models are also of relatively high density.
WO02/20261 describes a method of making models by making a sub-structure, applying a foamed mixed two-component resin (epoxy/amine or isocyanate/polyol systems) to the substructure to form a continuous layer, curing the resin and machining or hand cutting the cured resin to shape. This method is referred to as “net size casting” using a “seamless modelling paste” (SMP). The paste includes a thixotropic agent to increase the thixotropy of the paste after mixing and dispensing onto the substructure to ensure that the paste does not sag during curing. Amines are given as examples of suitable thixotropic agents.
2) Adhesives: In the aerospace, auto, rail, structural and other industries, two-part adhesives are widely used, e.g. in wind turbine blade bonding and to bond other structures. Thixotropic and gap filling adhesives are of special interest for successful bonding of large structures in order to achieve even, stress-free bonding, without flow out at the edges of the structures being bonded. Thixotropic high strength adhesives are also useful if they can be dispensed as ‘ropes’ onto vertical or slanting surfaces to adhere protective barrier panelling, e.g. on the sides of liquid gas tanks or fuel carriers
3) Component manufacture: Two-part curable resins are also used to form heavy electrical mouldings. Of special interest are flowable thermosetting compositions which can mix very well, set and cure evenly in the casings of large transformers.
4) Paints and coatings: Two-part curable resins are also used to form paints, e.g. automotive paints, and coatings and mouldings.
The above are given as examples of the use of two-component curable resins but the list is by no means exhaustive.
It is important that the individual components are flowable so that they can readily be mixed, especially when using machines that both mix and dispense the mixed composition. This sets certain limits on the viscosities that can be utilised and, in turn, sets limits on fillers and thixotropic agents that can be used, ultimately setting limits on the final properties that can be reached.
In many applications there is a need for the two-component composition to have a high viscosity shortly after mixing to provide a resistance to slump, i.e. a change in shape once the mixed composition has been placed in a desired location. The degree of non-slumping required can even be that of retaining almost exactly the shape and dimensions achieved by extruding the compositions through a shaped orifice. This non-slump texture is frequently obtained by dispersing a thixotropic agent such as a hydrophilic fumed silica in one of the components to blends, provided sufficient thixotropic agent is used, that generally retain their shape and non-slump properties until they are gelled and cured. A thixotropic composition can be defined as a composition whose viscosity under shear is lower than under no shear.
However, adding agents to increase the viscosity after mixing generally requires the individual components to also have high viscosities, even though they are thixotropic to a degree and hence have lower viscosities under shear than under no shear. The high viscosities of the components leads to difficulty in mixing the components together especially when mixing is achieved automatically during the dispensing of the mixture, leading to poor mixing of the components and hence a reduction in the properties of the cured resin.
This is especially true when using platelet nanofillers that increase the viscosity of compositions substantially, even at low loadings if highly dispersed.
Nanoparticles
Nanoparticles are particles of nanosize i.e. having at least one dimension on nanometer scale. They can be derived of naturally occurring- or synthesized-clay minerals, hence the name of nanoclays. Clays are generally phyllosilicates such as of the smectite group, for example a bentonite, montmorillonite, hectorite, saponite or the like. The surface of the clay can be modified to become organophilic hence the name of organoclays. The inorganic exchangeable cations which occur in natural or synthetic clay mineral are replaced by organic cations comprising sufficient carbon atoms to render the surface of the cation-exchanged clay hydrophobic and organophilic. For example U.S. Pat. No. 4,810,734 discloses phyllosilicates which can be treated with a quaternary or other ammonium salt of a to primary, secondary or tertiary organic amine in the presence of a dispersing medium.
Nanoclays are often plate-like materials also called platelets. Platelets have 2 dimensions higher than the third one; they have a planar extent and a thickness. Fibers have one dimension higher than the 2 others, no planar extent but a high length. Researchers have concentrated on four nanoclays as potential nanoscale particles (nanoparticles): a) hydrotalcite, b) octasilicate, c) mica fluoride and d) montmorillonite. The first two have limitations both from a physical and a cost standpoint. The last two are used in commercial nanocomposites. Mica fluoride is a synthetic silicate, montmorillonite (MMT) is a natural one. The theoretical formula for montmorillonite is:M+y(Al2-yMgy)(Si4)O10(OH)2*nH2O
Ionic phyllosilicates have a sheet structure. At the Angstrom scale, they form platelets, which can be 0.3, preferably 0.7 to 1 nm thick and several hundred nanometers (about 100-1000 nm) long and wide. As a result, individual sheets may have aspect ratios (Length/Thickness, L/T) varying from 200-1000 or even higher and, after purification, the majority of the platelets have aspect ratios in the 200-400 range. In other words, these sheets usually measure approximately 200×1 nm (L×T). These platelets are stacked into primary particles and these primary particles are stacked together to form aggregates (usually about 10-30 μm in size). The silicate layers form stacks with a gap in between them called the “interlayer” or “gallery”. Isomorphic substitution within the layers (Mg2+ replaces Al3+) generates negative charges that are counterbalanced by alkali or alkaline earth cations situated in the interlayer. Such clays are not necessarily compatible with polymers since, due to their small size, surface interactions such as hydrogen bonding become magnified. Thus, the ability to disperse the clays within some resins is limited and at the beginning, only hydrophilic polymers (e.g. PVA) were compatible with the clays because silicate clays are naturally hydrophilic. But, it was found that the inorganic cations situated in the interlayer can be substituted by other cations. Cationic exchange with large cationic surfactants such as alkyl ammonium-ions, increases the spacing between the layers and reduces the surface energy of the filler. Therefore, these modified clays (organoclays) are more compatible with polymers and form polymer-layered silicate nanocomposites. Various companies (e.g. Southern Clays (of 1212 Church Street, Gonzales, Tex. USA 8629), Süd Chemie, Nanocor, etc.) provide a whole series of both modified and natural nano clays, which are montmorillonites. Apart from montmorillonites, hectorites and saponites are the most commonly used layered silicates.
A nanocomposite is a dispersion, often a near-molecular blend, of resin molecules and nanoscale particles. Nanocomposites can be formed in one of the following three ways: a) melt blending synthesis, b) solvent based synthesis and c) in-situ polymerization, as is known in the art.
There are three structurally different types of nanocomposites: 1) intercalated (individual monomers and polymers are sandwiched between silicate layers); 2) exfoliated (a “sea” of polymer with “rafts” of silicate); and 3) end-tethered (a whole silicate or a single layer of a silicate is attached to the end of a polymer chain).
There has been immense activity in the use of nano clay composites in recent years, for use in polyolefins, methacrylates (e.g. PMMA), polyamides, bio-polymers, polyurethanes, phenols, polycarbonates, to achieve benefits and claims have been made for increase in strength, flame retardancy, barrier protection and high temperature resistance.
U.S. Pat. No. 6,579,927 details the formation of a nanomaterial where the clay material is homogeneously distributed throughout the polymeric matrix. The resultant nanocomposites could be moulded via injection moulding or extrusion processes.
Example 16 of the patent FR 1,452,942 discloses a two-part epoxy adhesive composition whose hardener part contains silica, hardener, carbon, and a silica aerogel whereas the resin part contains epoxy resin, bisphenol A and ammonium bentonite.
U.S. Pat. No. 6,197,849 details the preparation of organophilic phyllosilicates by treating naturally occurring or synthetic phyllosilicates with a salt of a quaternary or other cyclic amidine based compound. The patent covers polymeric systems, preferably epoxy resins, polyurethane and rubbers containing such organophilic phyllosilicates. The organophilic phyllosilicates may be added either to the resin or else to the hardener.
EP 0 267 341 A1 discloses a resin composition comprising smectite organoclays of improved dispensability. In an example, the organoclay is incorporated into component A of a two-pack Epoxy enamel.
EP 1 209 189 A1 discloses polymer foams containing nanoclay described as nanosized clay of plate-like form, dispersed therein. For example, clay platelet CLOISITE® 10A is dispersed in the polyol part of a polyurethane foam.
An article entitled “Polyurethane Nanocomposites Containing Laminated Anisotropic Nanoparticles Derived From Organophilic Layered Silicates” by Carsten Zilg, published in Advanced materials, VCH, Verlagsgesellschaft, Weinheim, Del., vol. 11, No. 1 07 Jan. 1999, pages 49-52, discloses a polyurethane nanocomposite material prepared from a polyol dispersion containing ion-exchanged organophilic fluoromica and an isocyanate component.
The incorporation of nano clay materials into polymer matrices, to enjoy the above-mentioned benefits, is not straight forward, however. The highly anisotropic nature and large surface area of nano clays can give problems in processing of polymers, particularly where 2 component reactive systems are envisaged. High loadings of the nano clay can result in unacceptably high viscosities, yet high viscosity is what is sought to achieve anti-slump characteristics in reactive systems.
A problem underlying the present invention is to develop two component systems where the components individually are of reasonably low viscosity for ease of processing, particularly for machine dispensed materials, yet which develop high viscosity when the components are mixed together to form a resin that is undergoing curing.
None of the above mentioned prior art documents provide a clue to solve that problem.